As a vibration damping device, a variable damping force damper has been known which uses a magnetorheological fluid (MRF) composition to change its apparent viscosity depending on the strength of a magnetic field applied to the MRF composition. This vibration damping device damps incoming vibration using flow resistance of the MRF composition which is caused when a piston reciprocates in a cylinder filled with the MRF composition. This vibration damping device is capable of controlling the vibration damping force depending on the strength of the applied magnetic field.
Meanwhile, as the MRF composition, what is obtained by dispersing magnetic particles into a dispersant is generally used. Thus, operating pressure inside the cylinder is higher in the vibration damping device using the MRF composition than in a vibration damping device using no MRF composition. Accordingly, the vibration damping device using the MRF composition involves a problem of an increase in frictional force between frictionally slidable portions. In addition, in the vibration damping device using the MRF composition, the existence of magnetic particles of the MRF composition between the frictionally slidable portions makes the frictional force larger between the frictionally slidable portions. Incidentally, an increase in the frictional force between the frictionally slidable portions, for example, makes ride comfort performance of a vehicle, to which this vibration damping device is applied, become worse, and makes the vibration damping device less adaptable to a light-weight vehicle.
A vibration damping device has been disclosed in which: a piston rod connected to a piston is slidably supported by an end portion of a cylinder with a bearing; and this bearing is surrounded by a porous member (see Patent Literature 1, for example).
This vibration damping device prevents the magnetic particles from existing between the frictionally slidable portion of the piston rod and the frictionally slidable portion of the bearing by supplying only liquid components of the MRF composition, which are obtained by filtering the magnetic particles from the MRF composition by the porous member, to the interstice between the frictionally slidable portions.
In addition, a magnetic fluid formulation has been also known which is prepared with re-dispersability and low-temperature fluid behavior of magnetic particles taken into consideration (see Patent Literature 2, for example). However, no lubricant composition which contributes to an increase in lubricity between the frictionally slidable portions, such as a decrease in frictional force between the frictionally slidable portions while the vibration damping device is in operation, has been known yet.